Field of the Invention
The present invention relates to a technique for detecting grammage of a recording material with high accuracy.
Description of the Related Art
Some of conventional image forming apparatuses such as a copier and a printer internally include a sensor for determining the type of recording material. These apparatuses automatically determine the type of recording material and controls transfer and fixing conditions according to the determination result. Examples of the transfer condition include a transfer voltage and a conveyance speed of the recording material during transfer. Examples of the fixing condition include a fixing temperature and a conveyance speed of the recording material at fixing.
Japanese Patent Application Laid-Open No. 2009-29622 discusses an image forming apparatus including an ultrasonic sensor that detects grammage of a recording material by transmitting an ultrasonic wave to the recording material and receiving the ultrasonic wave that has been attenuated after being transmitted through the recording material. The image forming apparatus controls image forming conditions such as transfer and fixing conditions according to the grammage of the recording material that has been detected by the ultrasonic sensor. For feeding back the detection result to the transfer conditions, the ultrasonic sensor is arranged on an upstream side, in a conveyance direction, of a transfer unit that transfers an image onto the recording material.
Meanwhile, there has been a known control of forming a loop on a recording material when an image is transferred onto the recording material by the transfer unit. More specifically, a conveyance unit is positioned on an upstream side of the transfer unit in the conveyance direction of the recording material. The conveyance unit conveys the recording material to the transfer unit at a speed faster than that of the transfer unit, so that a loop is formed on the recording material by the conveyance unit and the transfer unit. The reason for forming a loop on the recording material is that if the recording material is pulled from the conveyance unit while an image is being transferred onto the recording material by the transfer unit, the speed of the recording material may vary during the transfer to cause a problem in the image being transferred. On the other hand, if a loop is formed on the recording material, a significant change may occur in the orientation of the recording material to affect a detection result of the ultrasonic sensor. If the image forming conditions are set based on an erroneous detection result, image quality may be degraded.